1. Field of the Invention
The present invention relates generally to passive microwave structures.
2. Description of the Related Art
Intermodulation distortion results from nonlinearities in the transmission function of an electronic device and is characterized by the appearance of signals in the device output whose frequencies are equal to the sums and differences of the input signal frequencies. In particular, if two signals having frequencies f.sub.1 and f.sub.2 are processed through a nonlinear device, e.g., a diode or a transistor, the device output will include intermodulation products whose frequencies are nf.sub.1 .+-.mf.sub.2, in which n and m are integers.
Because these intermodulation products are spread across a wide frequency range, some of them often fall in a frequency band of interest, e.g., third order products (in which n+m=3) lie close to amplified versions of the input signals and are therefore difficult to remove by filtering. Accordingly, extensive efforts have been directed to reducing the presence of intermodulation products in signal bands (e.g., by reducing the amplitude of the products and/or by properly positioning the frequency of the products in relation to the signal band).
It was once thought that intermodulation products were only produced by active devices. However, it has been found that passive microwave structures which are generally considered to be linear, e.g., filters, waveguides, waveguide-based components (such as couplers) and enclosures, also exhibit intermodulation distortion. Because it is generated in passive microwave structures, this type of intermodulation distortion is typically referred to as passive intermodulation (PIM).
Various investigations (e.g., see Chapman, R. C., et al., "Intermodulation Generation in Normally Passive Linear Components; Section 1.0--IM Generation", US Army Satellite Communications Agency Study Report WDL-TR5242, Aug. 24, 1973) have discovered that the sources of PIM include a) microdischarge at contacting points of metallic joints, b) nonlinear contact impedance due to oxide layer tunneling, c) space charge limited current flow through oxide films and d) water vapor absorptive and dispersive effects.
It has been found that PIM effects are reduced when metallic members of microwave passive structures are coupled by the joining processes of welding or brazing. However, welded and brazed parts cannot easily be assembled and disassembled. In addition, welded joints are susceptible to developing cracks which are PIM generators and it is difficult to control warpage in brazed assemblies.
It has also been shown (e.g., see Rootsey, J. V., et al., "Intermodulation Study (Intermodulation Products--Satellite Ground Antennas); Section 1.0--Introduction and Section 2.0--Conclusions and Recommendations", US Army Satellite Communications Agency Study Report AD-785711, Aug. 24, 1973) that another joining process in the form of bolted joints reduces PIM when these joints include a) the use of relatively soft materials, e.g., aluminum, b) mating surfaces which are sufficiently smooth, c) mating parts which are sufficiently thick to reduce bending and d) bolt spacing which is sufficiently small. In general, these bolted joints reduce PIM by inducing stresses in opposing surfaces that are sufficient to reduce PIM generating sources, e.g., intersurface oxide films. Unfortunately, bolted joints increase the weight, size, fabrication cost and assembly time of passive microwave structures.